The present invention relates to an engine preheating apparatus aimed at a rapid temperature rise characteristic of a glow plug.
Many conventional Diesel engines are provided with a glow plug for facilitating the engine starting. At the time of engine start, this glow plug is heated to become red-hot and fuel is brought into contact therewith and burnt thereby to facilitate the engine start. The time period required for heating the glow plug to red-hot state (hereinafter referred to as the glow plug preheating time) and the time required for starting the engine by turning the starting motor (hereinafter referred to as the cranking time) are preferably as short as possible. In view of these requirements, the present applicant filed U.S. patent application Ser. No. 195,504, now U.S. Pat. No. 4,317,434, suggesting a method of providing rapid temperature rise characteristic to the glow plug.
Specifically, the apparatus using such a method comprises a glow plug mounted on the engine, an actuation circuit for the glow plug, a starting resistor of barium titanate connected in the middle of the actuator circuit and having a positive temperature coefficient of resistance so that the resistance value thereof suddenly increases at a specific temperature, and a normal-operating resistor connected in parallel to the starting resistor. The resistance value and the heat capacity of the starting resistor are determined in such a manner that when the glow plug circuit is energized, the starting resistor has a resistance value smaller than the normal-operating resistor as long as the glow plug is low in temperature, and the resistance value of the starting resistor exceeds that of the normal-operating resistor at higher temperatures. Further, by decreasing the resistance value of the glow plug and thereby reducing the rated voltage, the rapid temperature rise at low temperatures of the glow plug and the prevention of the breakage of the glow plug by excessive heat at high temperatures of the glow plug are attained at the same time.
When the engine is started and stopped repeatedly, or especially when the engine fails to be started and preheated again several tens of seconds later, however, the temperature rise of the glow plug is likely to be retarded to some degree. This is by reason of the fact that the time constant for cooling the glow plus is smaller than that for cooling the starting resistor. In other words, when the repeated preheating is required several tens of seconds after a starting error, the glow plug is cooled, and in spite of the requirement to restore the starting resistor to normal temperature, it remains at high temperature because of the large time constant thereof. Since the starting resistor remains at high temperature with a high resistance value, current flows through the normally-operating resistor, so that a high voltage fails to be applied to the glow plug, resulting in a slow temperature increase.